1. Field of the Invention
This invention relates to a brake device for a shutter which is arranged to apply the brake to the shutter when a group of shutter blades nearly come to the end of their travel.
2. Description of the Related Art
As a result of a desire to enlarge the region of photography, cameras have recently come to have a highest shutter speed of 1/4000 sec and an X-sync shutter speed of 1/250 sec. In attaining such a high shutter speed, the weight of the shutter blades is reduced and a blade driving spring force is increased for a higher travelling speed of the shutter blades.
The absolute strength of the shutter blades is decreased by the reduction in weight. Despite that, the blades must be driven to travel by an increased driving force. Particularly, at the end of the travel at which the blades achieve their maximum travelling speed, the brake is abruptly applied with a short stroke for bringing them to a stop at a predetermined point. Therefore, a very large force is applied to the blades. Hence, the durability of the blades greatly depends on the brake to be applied at the end of the travel.
In the case of the conventional brake device which is of the kind to obtain braking by friction and is popularly used for the above-stated purpose, the peak of the braking force is obtained at the beginning of the operation of the brake device. However, the slit forming edge of a slit forming trailing blade still remains at the edge of the aperture of the shutter when the brake device beins to operate. Therefore, the blade is warped by the impact of a sudden drop in speed caused by the brake application. This tends to break the shutter with the shutter blade colliding against the aperture edge of the shutter. The breakage of the shutter blade due to such collision is avoidable to a certain degree by initiating brake application after the trailing shutter blade has passed the edge part of the shutter aperture. However, such an arrangement reduces the braking stroke too much for obtaining a sufficient braking effect and also decreases the durability of the shutter.
A brake device which is directed to a solution of the above stated problem is disclosed in U.S. Pat. No. 4,692,009. The brake device is arranged to have first and second brake levers; to apply the brake with a relatively weak force by means of the first lever in the first half of the braking stroke; and to apply brake with a stronger braking force by using both the first and second lever in the latter half of the stroke which takes place after the slit forming shutter blade has passed the edge part of the shutter aperture. However, since the braking force changes by two steps in that instance, the braking force inevitably has two peaks, one at the beginning of the first step and the other at the beginning of the second. That device thus does not permit smooth braking. Therefore, although the brake device is a great improvement over the conventional brake device, the durability of the shutter blades is still affected by that brake device. Although it has been possible to adjust the charateristic curve of the braking force by changing the engaging timing of the first and second levers and the pressed contact pressure of the brake lever on a friction seat, the possible degree of such adjustment has been insufficient.
Meanwhile, a brake device of another type is disclosed in Japanese Utility Model Application Laid-Open No. SHO 60-70832. This brake device is arranged to continuously increase the braking force in proportion to the operating degree of a brake lever. What is claimed by that Japanese utility model application is as follows: "A brake device for a shutter characterized in that: a fixed base plate or an auxiliary member which is guided to be movable only perpendicularly to the base plate and a brake operating member which is arranged to move in parallel to the base plate are arranged to overlap each other; these members are pushed in the perpendicular direction by means of a spring into contact with the base plate through a slanting frictional face; and a braking force which is applied while shutter curtain blades are travelling is gradually increased by changing the pushing pressure of the spring by moving perpendicularly either the brake operating member or the auxiliary member through the action of the slanting face according to the motion of the brake operating member."